Angel of the Lord is a messenger of Yahweh, which at times would appear to be a regular created angel, and at other times the eternal non-created Son of God.

Angel of the Lord
David Cox’s Topical Bible Concordance

The term “Angel of the Lord” refers to an angel which pertains to, or is in the service of “the Lord”. This is opposed by those who walk in rebellion to God. Note that in Hebrew, when two nouns are links x of y, if the y the definite article or is a name of a person, the the “angel” is also definite, i.e. there is no way to differentiate between “an angel of the Lord” and “the angel of the Lord”.
New Testament references
Mat. 1:20,24; 2:13,19; 28:2; Luk. 1:11; Act. 5:19; 7:30,35; 8:26; 12:7,23.
– Called Angel of God Exo. 14:19; Judg. 13:6; 1Sa. 29:9; 2Sa. 14:17,20; 2Sa. 19:27; Act. 27:23; Gal. 4:14.
– Called Angel of the Lord Gen. 16:7,9; 22:11; Exo. 3:2; Num. 22:23,25,27,32,35; Judg. 2:1; 6:11-12,21-22; 13:3,6,9,13-21; 2Sa. 24:16; 1Ki. 19:7; 2Ki. 1:3,15; 2Ki. 19:35; 1Ch. 21:15,18; Psa. 34:7; 35:5-6; Zec. 1:11-12; 3:5; 12:8.
– Called Angel of His Presence Isa. 63:9.